generic convenience function to convert logbf matrix to PP matrix
Arguments
- bf
an L by p or p+1 matrix of log Bayes factors
- pi
either a scalar representing the prior probability for any snp to be causal, or a full vector of per snp / null prior probabilities
- last_is_null
TRUE if last value of the bf vector or last column of a bf matrix relates to the null hypothesis of no association. This is standard for SuSiE results, but may not be for BF constructed in other ways.